The present invention relates to NCO prepolymers having a low monomer content and prepared from isophorone diisocyanate (IPDI) and polypropylene oxide glycols, to a process for the production thereof and to their use.
Due to their good lightfastness and weathering resistance, aliphatic and cycloaliphatic diisocyanates are used in high quality polyurethane coatings, sealants and embedding compositions (see for example Wagner Sarx, xe2x80x9cLackkunstharzexe2x80x9d [synthetic coating resins], 5th edition, Carl Hanser Verlag, Munich, 1971, pages 153-173). As a consequence of increased environmental awareness, solvent-free systems have in particular attracted interest. One possible option for producing such systems is to use NCO prepolymers. A multitude of NCO prepolymers and binders obtained therefrom are known (for example EP-A 0 497 131 or EP-A 1 138 707).
Prior art NCO prepolymers are produced by reacting relatively high molecular weight polyhydroxyl compounds, such as polyether or polyester polyols, with excess quantities of di- or polyisocyanate. It is advantageous in this connection if the isocyanate groups exhibit different reactivities. This property of xe2x80x9cselectivityxe2x80x9d suppresses any unwanted monomer content. These products also exhibit low viscosities and better industrial processability.
Isophorone diisocyanate (3-isocyanatomethyl-3,3,5-trimethylcyclohexyl isocyanate, IPDI) has two such differently (selectively) reactive isocyanate groups. However, it is not straightforwardly possible to reduce the residual monomer content of a prepolymer produced from IPDI to below 2.0 wt. % (for example EP-A 0 497 131, Example 2, or EP-A 1 138 707, Comparative Example). To further reduce the monomer content, it is generally necessary to perform a subsequent, technically elaborate distillation step (thin film distillation process) in order to separate monomeric constituents, for example IPDI.
Another option is described in DD-A 151466, in which prepolymers having a low monomer content are produced by reacting di- or polyisocyanates with polyalcohols using hydroxy-functional organometallic catalysts. However, these prepolymers can only be produced with the use of solvents, some of which remain in the prepolymer. This is disadvantageous for reasons of occupational hygiene, especially when these prepolymers are subsequently used in paints for producing coatings in poorly ventilated areas. Also, a low residual monomer content of diisocyanate according to DD-A 151466 is only achieved by using elevated contents of hydroxy-functional organometallic catalysts. This means, however, that considerable proportions of isocyanate are consumed by reaction with the hydroxy function of the catalyst and are not available for further polymer synthesis. This chain-terminating function of the catalyst also has a disadvantageous effect on, for example, mechanical properties of the paints formulated from the prepolymers.
An object of the present invention is to provide solvent-free, NCO prepolymers prepared from IPDI and polyhydroxyl compounds and having a residual IPDI monomer content of  less than 2.0 wt. %, which prepolymers should be directly usable without a distillation process for removing the monomers and do not exhibit the stated disadvantages.
It has surprisingly been found that this object may be achieved by using polypropylene oxide glycols, which have been produced using the DMC catalyst process (for example, U.S. Pat. No. 3,278,459, WO 97/29 146 or EP-A 0 573 206). Without any elaborate distillation step, the monomer content is less than 2.0 wt. % and thus lower than when conventional polypropylene oxide glycols produced using the KOH process are used.
The present invention relates to solvent-free, NCO prepolymers having a residual IPDI monomer content of less than 2.0 wt. %, an NCO content of 2.0 to 5.0 wt. % and a viscosity of 5000 to 20,000 mPaxc2x7s, wherein the NCO prepolymers are the reaction products of
i) 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate and
ii) one or more polyethers prepared by the DMC catalyst process and having an OH-functionality of 1.80 to 2.00
at an NCO/OH equivalent ratio of 1.5:1 to 2.2:1.
The present invention also relates to a process for preparing these solvent-free, NCO prepolymers by reacting
i) 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate and
ii) one or more polyethers prepared by the DMC catalyst process and having an OH-functionality of 1.80 to 2.00
at an NCO/OH equivalent ratio of 1.5:1 to 2.2:1 in the presence of at least one catalyst at 20 to 100xc2x0 C.
Finally, the present invention relates to polyurethane coating, sealant, adhesive and/or embedding compositions containing the solvent-free, NCO prepolymers as a binder and/or binder component.